1. Field of the Invention
This invention relates to a display apparatus employing a field emission type cathode, and a method for driving the same.
2. Description of the Related Art
There has recently been developed a display employing, for example, a field emission type cathode as one of flat panel shaped display units employed for a display apparatus. As a display employing this field emission type cathode, there is known a field emission display (FED). The FED has many features, such as high picture quality and production efficiency, quick response speed, operability under extremely low temperature environments, high luminosity and high power efficiency. In addition, the FED can be produced by a simpler process than that for production of the so-called active matrix liquid crystal display. The production cost for the FED is expected to be lowered at least to as much as 40 to 60% of that of the active matrix type liquid crystal display.
Referring to FIGS. 1 and 2, the basic structure and the operating principle of the FED is explained.
FIG. 1 shows the basic structure of the FED. In this figure, a cathode emission section 50 includes a glass substrate 10, a cathode electrode 5, an insulator 4, a gate electrode 3 and a cathode 6. On a glass substrate 10 of the electron emission section 50 are layered the cathode electrode 5, insulator 4 and the gate electrode 3. On the glass substrate 10 is formed the cathode electrode 5 which is insulated from the gate electrode 3 by the insulator 4. The insulator 4 and the gate electrode 3 are formed with plural openings within which cathodes 6 for enhancing the intensity of the electrical field are arranged on the cathode electrode 5. The cathodes 6 and the cathode electrodes 5 are electrically connected with each other. The cathode electrodes 5 and the cathodes 6 make up a field emission type cathode. Facing the surface of the gate electrode 3 of the electron emission section 50 is arranged a light emitting section 51. That is, the light emitting section 51 is arranged in a direction along which electrons 7 are emitted from the cathode 6, as will be explained subsequently. The light emitting section 51 is comprised of a glass substrate 9 on which is layered an anode electrode 1 formed by a transparent material, such as indium tin oxide (ITO). A phosphor element 2 is coated on the surface of the anode electrode 1 facing the glass substrate 9. The surface of the phosphor element 2 faces the front surface of the gate electrode 3 of the electron emission section 50. The spacing between the electron emitting section 50 and the light emitting section 51 is maintained at vacuum. A plurality of the cathodes 6 are associated with a single pixel (phosphor element) (phosphor element 2), with the focal point of each cathode 6 being on the associated phosphor element 2. Thus, by applying an electrical voltage across the gate electrode 3 and the cathode electrode 5 of the electron emission section 50, electrons 7 are emitted from the electron emission section 50. In addition, by applying an electrical voltage across the anode electrode 1 of the light emission section 51 and the cathode electrode 5 of the electron emission section 50, the electrons 7 emitted are attracted towards the anode electrode 1 and collided against the phosphor element 2 of the light emitting section 51 for emitting light from the phosphor element 2. FIG. 1 shows an embodiment wherein the light emission section 51 is made up of three portions associated with three prime colors of red (R), green (G) and blue (B). The phosphor elements 2 emit the light in these three color R, G and B for realization of a color display.
Referring to FIG. 2, showing a portion of FIG. 1, the principle of driving of the field emission type cathode, employed in FED, is explained.
In FIG. 2, if a voltage Vk by a variable voltage source 53 and a voltage Vg by a variable voltage source 54 are applied to the cathode electrode 5 and the gate electrode 3, respectively, for applying a voltage difference represented by a voltage Vgk across the gate electrode 3 and the cathode electrode 5, the electrons 7 are emitted from the cathode 6 under an electrical field produced by such voltage application. If a voltage Va is applied by the variable voltage source 55 to the anode electrode 1, the electrons 7 are attracted towards the anode electrode 1 under a condition of EQU Va&gt;Vg (1)
so that an anode current Ia flows in a direction indicated by arrow ar in FIG. 2. If the phosphor element 2 is pre-coated on the anode electrode 1, the phosphor element 2 emits light under the energy of the electrons 7. The amount of the electrons 7 is changed with the voltage Vgk so that the anode current Ia is also changed. On the other hand, the amount of light emission of the phosphor element 2, that is luminosity L of the emitted light, is related with the anode current Ia by EQU L.varies.Ia (2)
so that, by changing the voltage Vgk, the value of luminosity L of the emitted light can also be changed. Thus the conventional practice has been to achieve luminosity modulation by modulating the voltage Vgk in accordance with the signal to be displayed. That is, with the above-described method for driving the field emission type cathode, the voltage Vg of the variable voltage source 54 is changed in accordance with the signal to be displayed for changing the voltage Vgk (driving voltage) for realization of luminosity modulation.
Meanwhile, the field emission type cathode has characteristics as shown in FIG. 3, from which it may be seen that the relation between the driving voltage Vgk and the anode current Ia (field emission current) is not linear but exponential. That is, the difference voltage between the gate electrode and the cathode electrode, that is the driving voltage, is not changed in proportion to the anode current (field emission current) Ia.
However, since the relation between the luminosity L and the anode current Ia is as shown by the above formula (2), so that, for driving the conventional display employing the field emission cathode, it becomes necessary to employ a correction circuit for setting the relation of proportionality between the voltage Vg and the luminosity L, as in the case of the gamma correction (non-linear correction) of a cathode ray tube.
The FED is comprised of gate electrodes 3a of plural lines, corresponding to the gate electrodes 3, and a cathode electrode 5a corresponding to the cathode electrode 5, arrayed in a matrix configuration. At an intersection of the gate electrode 3a and the cathode electrode 5a, that is at a pixel, a plurality of field emission cathodes 6a are arrayed as shown in FIG. 4B in which is shown enlarged a portion of FIG. 4A. If characteristics of the field emission cathodes 6a exhibit variations as shown in FIG. 5, variations in luminosity due to variations in characteristics of the field emission cathodes 6a are produced. That is, inherent characteristics a of the field emission cathodes 6a undergo fluctuations as shown by characteristics b or c. The above-described compensation circuit is necessitated for compensating these variations in the characteristics of the field emission cathode 6a.
In addition, the above-described field emission cathode poses a problem in that the field emission current (anode current Ia) cannot be feed back to the driving voltage (voltage Vgk) while cathode instabilities cannot be absorbed. As for these inconveniences, that is that the field emission current cannot be fed back to the driving voltage and cathode instabilities cannot be absorbed, it has been reported that these inconveniences may be evaded by a high electrical resistance connected in series with the cathode electrode. In such case, however, problems are raised that the response speed of the cathode is retarded and additional production steps need to be included in the production process.